The invention relates to rotary drill bits for use in coring holes in subsurface formations and of the kind comprising a bit body having a shank for connection to a core barrel, an outer peripheral gauge portion for engagement with the peripheral wall of the hole being drilled, an inner peripheral gauge portion defining at least a part of a central, generally circular aperture extending axially through the bit body, and a plurality of cutting elements mounted at the surface of the bit body.
Although not essential to the present invention, the cutting elements may be in the form of so-called "preform" cutting elements, each element being in the shape of a tablet, usually circular, having a hard cutting face formed of polycrystalline diamond or other superhard material.
The core barrel for use with the bit may be of the known kind which includes an inner wall spaced inwardly from the outer wall of the barrel, a bearing being provided between the two walls so that the inner wall may remain stationary around the core being cut while the main outer wall of the barrel rotates with the bit. The inner wall may carry at its lower end a "core catcher" operable to engage the core being cut adjacent its lower end, when required, so that as the core barrel is subsequently lifted the core breaks in the vicinity of the core catcher and is lifted within the core barrel.
During drilling the cutting elements are normally cleaned and cooled, and cuttings are removed, by pumping drilling fluid down the drill string, into the core barrel and over the surface of the bit. In one common form of bit, the inner gauge of the bit body is formed with axially extending channels so that drilling fluid may reach the outer surface of the bit body from the interior thereof by flowing downwardly through such channels. However, in this case the drilling fluid passes over the surface of the core being cut. This causes erosion of the core which may lead to premature core breakage, difficulties in removing the core from the hole and loss of geological information.
In an effort to overcome such erosion problems, some coring bits are provided with so-called "face-discharge" in which passages for drilling fluid are formed within the bit body itself to permit drilling fluid to flow from the interior of the core barrel to a nozzle in the surface of the bit body. In such arrangements the inner surface of the gauge portion is still often relieved by channels or fluting, although face-discharge bits are also known having a plain inner peripheral gauge unrelieved by channels or fluting.
With known face-discharge coring bits, although the major portion of the flow takes place through the passages in the bit body, there is till a significant amount of flow between the inner gauge and the surface of the core. Since the amount of erosion caused is dependent largely on the velocity of flow over the surface of the core rather than the volume flow rate, and since the velocity of flow is dependent on the pressure drop, it is believed that conventional face-discharge bits do not significantly reduce erosion of the core.
In some designs of core barrel for use with a face-discharge bit, the core catcher is extended and shaped to deflect drilling fluid away from the core. This is however largely ineffective because it still permits some flow of fluid past the core with the consequent erosion which occurs for the reasons mentioned above.
In view of these erosion problems, therefore, it is normally considered necessary to use a much lower pressure drop in the drilling fluid when drilling when a core bit, when compared with other types of full-hole bit. For example, in a full-hole bit the pressure drop might be in the range of 500 to 1200 psi, and occasionally as high as 2000 psi, whereas in core drilling the pressure drop is more likely to be in the range of 50 to 200 psi, although occasionally it may be as high as 400 psi. The present invention sets out to provide an improved form of coring bit and method in which core erosion is reduced or eliminated and which thus permits the use of a greater pressure drop across the coring bit with consequently greater velocity of flow of drilling fluid. This may give better cleaning and cooling of the cutting elements and less risk of blocking of flow channels in the surface of the bit body.